Reinventing Discovery - Michael Nielsen [50]
How do you go about making a map of the universe? It’s a surprisingly complicated problem. Ideally we’d like a map to show both objects that are relatively close in astronomical terms, such as the nearby stars, which are just a few light-years away, and also the most distant galaxies, which are billions of light-years away. It’s hard to do both those things on the same map. The mapmaking problem is o complicated by the fact that the universe is three-dimensional, while ordinary maps are two-dimensional. Of course, there are many ways you can try to address these complications, but that leads to still another problem: of the many ways you can make your map, which way is the best? A feature that’s strikingly obvious in one way of visualizing the universe may be nearly invisible in another. And what if you make the wrong choice? Mapping the Earth’s surface is a much easier problem, yet early mapmakers still tried out many different projections to make sense of the Earth. Similarly, Gott and his collaborators experimented with many different ways of making their map. One of the maps they made took the galaxy data from the SDSS, and used it to visualize the distribution of galaxies in the universe. That map is shown in figure 6.1. It’s not an ordinary map like a roadmap, and so it takes a bit of effort to understand, but it’s worth reading through the caption in detail to understand what’s being shown. The key point is the concentration of galaxies in the upper left-hand corner of the map, a concentration much denser than through the rest of the map. It was humanity’s first ever glimpse of the Sloan Great Wall.
The Sloan Great Wall is just one of thousands of scientific discoveries made using the SDSS. To give you more of the flavor of the SDSS’s impact, let me briefly describe two more of those discoveries. You perhaps already know that our Milky Way galaxy has two neighboring galaxies, the Large and Small Magellanic clouds. These are dwarf galaxies, with the larger of the two containing about 30 billion or so stars, compared to our Milky Way’s hundreds of billions. If you’ve never been to the southern hemisphere, then you may never have seen the Magellanic clouds, for they’re too far south in the sky to be visible from much of the northern hemisphere. But they are visible on a dark night in the southern hemisphere, where they show up as smudges in the sky. According to our best current understanding of galaxy formation, the Milky Way should have tens or hundreds of nearby dwarf galaxies. But prior to the SDSS only a few dwarf galaxies other than the Magellanic clouds had been discovered, and it was a puzzle where all the other missing dwarfs were. When the SDSS images became available, several astronomers searched the images for more dwarf galaxies. They didn’t do this manually—it would have taken far too long to peruse all the images. Instead, they used computer algorithms to search out new dwarf galaxies in the SDSS images. What they’ve found so far is nine new dwarf galaxies near the Milky Way, going much of the way toward solving the puzzle of the missing dwarfs.
Figure 6.1. A blown-up piece of one of the maps of the universe made by Gott and collaborators. You’ll notice that the map resembles a piece of pie. You should imagine yourself on the Earth, right at the center of the pie, looking out at the universe. Each point on the map represents a single galaxy from the SDSS. The radial direction indicates the distance to the galaxy, with the closest galaxies in the plot about 700 million light-years away, and the furthest about 1,300 million light-years away (as marked on the right-hand side). All the galaxies shown in the plot are very close to the celestial equator, the great arc going across